Which of these doesn't belong?

One of these reels is not an 8mm film. It's 16mm leader.

The fourth reel from the left is quite an odd sight to see. That's because it's a leader of 16mm film - cut in half. This is something that was sometimes done with double perforated 16mm leader but never with regular perforated leader. The leader cannot be projected as it is, because every second perforation hole is missing.